1. Field of the Invention:
The present invention relates to a dynamic focus circuit suitable for use in a cathode-ray tube for deflecting a large screen size and a wide angle or a thick-neck type cathode-ray tube having a large-diameter electron lens, and to a display using the dynamic focus circuit, which includes a television receiver, etc.
2. Description of the Related Art:
In a cathode-ray tube employed in a television receiver or the like, the distance of an electron beam to the center portion of the screen thereof generally differs from that thereof to its corner portion. Therefore, even if the electron beam is adjusted to the most satisfactory focus at the central portion of the screen thereof, the electron beam overconverges on the corner portion if left as it is. This phenomenon significantly occurs in, particularly, a display including a television receiver or the like using a cathode-ray tube for deflecting a large screen size and a wide angle or a thick-neck type cathode-ray tube having a large-diameter electron lens.
Therefore, in this type of display, a dynamic focus voltage varied according to the position of deflection of the electron beam is normally produced by a dynamic focus circuit. The produced dynamic focus voltage is applied to a focus electrode of the cathode-ray tube. This has provided compensation for focusing so as to maintain the electron beam at the most suitable focus over the entire screen.
However, there has been a demand for increasing the dynamic focus voltage to a further high voltage with a great increase in the screen of a television receiver or a display or the like, which has been used in recent years, the widening of a deflecting angle employed therein and a great increase in diameter of an electron gun employed therein. The value of such a voltage has been rendered very high.
To meet such a demand, a method of generating a high voltage using a transformer and a method of generating a high voltage using a high-voltage power supply and active elements such as a transistor, etc. have heretofore been used by way of example. As a prior art related to the former method, may be mentioned Japanese Patent Application Laid-Open No. 3(1991)-165665, for example. Further, Japanese Patent Application Laid-Open No. 3(1991)-3961 is known as a prior art related to the latter method.
However, the methods according to the prior arts described above have the following problems.
According to the former method, i.e., the method of boosting the voltage using the transformer to obtain the dynamic focus voltage, a necessary high dynamic focus voltage can be obtained by increasing a turn ratio of the transformer. However, the core of the transformer increases in size correspondingly. Therefore, the present method has a drawback that it is necessary to increase the area of a circuit board and the cost of materials for the transformer becomes high.
Further, the latter prior art also has a drawback that since a power supply whose supply voltage is further higher than that employed in the prior art, is required to obtain a dynamic focus voltage whose amplitude is made greater, the cost of the power supply rises in the same manner as described above. The present prior art is also accompanied by a drawback that since a absolute maximum voltage characteristic of a transistor element capable of withstanding a high voltage is required with the increase in the voltage of the power supply, the cost of the circuit rises in the same manner as described above. It is also necessary to make a circuit contrivance or the like for ensuring a through rate of the circuit according to the increase in the voltage of the power supply. In addition to this, a problem arises that the operation (particularly, such as the amplitude of an output thereof, the D.C. potential of the output thereof) of an amplifier circuit becomes unstable due to external factors such as a change in temperature, etc.